1. Field of the Invention
There is an increasing number of genes being introduced into foreign hosts for expression. A substantial proportion of the genes which are employed are obtained from cDNA. In those instances where the natural gene has one or more introns, it has frequently been found to be much more convenient to employ the cDNA rather than handling larger pieces of DNA which involve the intron and add further complexity to the manipulation of the gene in conjunction with the regulatory signals associated with expression and replication.
The function of introns is still not well established. While much progress has been made in an understanding of the splicing mechanism involved in removing the introns, the role of introns in the process of transcription and translation has not been explained. The fact that cDNA can be readily expressed establishes that introns are not essential for the expression of a coding sequence.
An important factor in the preparation and production of polypeptides and proteins in cellular hosts is the efficiency with which the polypeptide or protein is produced. Since nutrients must be supplied to the host to maintain its viability, the nutrients are used primarily for the growth and replication of the host, rather than the product of interest. It thus becomes important to maximize utilization of the nutrients for the production of the desired product, as well as enhancing the proportionate amount of the desired product as compared to the total protein produced, which greatly aids in the efficiency with which the desired product may be isolated in pure form.
2. Description of the Prior Art
Pennica et al., Nature (1983) 301:21-221; UK Patent Appln. No. GB 2,119,804 describe the isolation and cloning of a cDNA for human tissue plasminogen activator. Ny et al., Proc. Nat'l Acad. Sci. USA (1984) 81:5355-5359 describe the human tissue-type plasminogen activator gene and the exon-intron relationship. Hamer and Leder, Cell (1979) 18:1299-1302, suggest that introns aid in the stability of RNA, while Gething and Sambrook, "The Expression of the Influenza Virus Hemagglutinin Gene from SV-40-HA Recombinants" in Eukaryotic Viral Vectors, ed., Y. Gluzman, pp. 29-33 (1982), indicate the opposite experience. Kaufman and Sharp, Molec. and Cell Biol. (1982) 2: 1304-1319 show that a hybrid intron can be used in the expression of dihydrofolate reductase cDNA. Lau and Kan, Proc. Nat'l Acad. Sci. USA (1983) 80:5225-5229, describe cosmid vectors and their use in COS cells and permanent cell lines.